We have been studying the role of the immune response to gut flora in the development of Inflammatory Bowel Disease (IBD). We previously demonstrated that experimental infection with a single bacterial agent, Helicobacter hepaticus, induces chronic colitis in specific pathogen-free IL-10 KO mice and that the disease is accompanied by a Type 1 cytokine response detected by restimulation of T cells with a soluble H. hepaticus Ag preparation. In contrast, wild-type (WT) animals infected with the same bacterium did not develop disease, and produced IL-10 as the dominant cytokine in response to Helicobacter Ag. In the current year, we investigated the phenotype and specificity of the CD4+ T cells from IL-10 deficient mice that induce IBD as well as the T cells that prevent disease in WT animals. The biological activity of these cell poulations was assayed by transfer into T deficient RAG-2 KO mice. Purified CD4+ T cells as well as 3 individual CD4+ T cell clones derived from infected IL-10 KO mice were shown to transfer colitis to RAG-2 recipients but only when these animals were also infected with H.hepaticus.The clones have classical alpha/beta T cell receptors and appear to recognize membrane associated H. hepaticus antigen(s). In parallel studies CD4+ T cells from infected WT mice were shown to protect RAG recipients against the colitis induced by co-transferred IL-10 KO T cells. The protective CD4+ T cells were shown to display a CD45RBlo phenotype but did not require the CD25 marker associated with regulatory T cells in other systems. These findings argue that IBD can be triggered by bacterial specific CD4 T cells but that this pathologic response is normally prevented by CD4+ regulatory cells which also appear to recognize bacterial antigens. In a separate project we investigated the role of IL-10 produced by antigen presenting cells (APC) in regulating host resistance to Mycobacterium avium infection. When infected with this pathogen, transgenic mice designed to express human IL-10 only in APC developed significantly larger bacterial loads and more fibrotic granulomas (containing large numbers of giant cells) than did WT controls. The observed increase in susceptibility correlated with suppressed production of pro-inflammatory mediators associated with macrophage activation. Interestingly, bacterial burdens continued to rise in these animals despite the eventual down-regulation of IL-10 transgene expression. The latter observation supports the concept that early immunoregulatory changes can have a long-term influence on the outcome of mycobacterial infections.